Modified polytetrafluoroethylene fine powder and method for the production of the same

ABSTRACT

A modified polytetrafluoroethylene fine powder containing agglomerates of colloidal particles with an average particle size of from 0.05 to 0.5 μm which comprise a copolymer of tetrafluoroethylene and an olefin-monomer represented by X(CF 2 ) n OCF═CF 2  in which X is a hydrogen atom, a fluorine atom or a chlorine atom, and n is an integer of 1 to 6, and/or C 3 F 7 (OCF 2 CF 2 CF 2 ) m [OCF(CF 3 )CF 2 ] p OCF═CF 2  in which m and p are independently an integer of 0 to 4, provided that the sum of m and p is not 0 (zero), a content of the olefin monomer being from 0.02 to 0.3 wt. %, wherein the copolymer has a molecular weight distribution Mw/Mn of from 1.5 to 4.5 and a standard specific gravity of from 2.135 to 2.175. This modified polytetrafluoroethylene has good paste extrudability, and provides molded articles having good mechanical properties, in particular, pressure resistance.

This application is the national phase under 35 U.S.C. §371 of prior PCTInternational Application No. PCT/JP97/02742 which has an Internationalfiling date of Aug. 7, 1997 which designated the United States ofAmerica.

FIELD OF THE INVENTION

The present invention relates to a modified polytetrafluoroethylene finepowder having good mechanical properties, in particular, pressureresistance, and a method for the production of the same.

PRIOR ART

Polytetrafluoroethylene (PTFE) for molding roughly includes two types ofpowders, that is, a powder which is obtained by finely pulverizinggranular resins prepared by suspension polymerization (granular resin),and a powder obtained by coagulating polymers from latexes prepared byaqueous dispersion (emulsion) polymerization and drying the coagulatedpolymers (fine powder), and both powders are practically used. These twotypes of the powders are processed by entirely different moldingmethods. For example, the former powder is processed by compressionmolding or ram extrusion molding, while the latter powder is processedby paste extrusion molding which is carried out by compounding liquidlubricants in the powder, or calender molding. Such PTFE is supposed tohave a very high molecular weight, and cannot substantially bemelt-processed. Thus, the above-described special molding methods areemployed.

Powders of low molecular weight PTFE are called “waxes”, anddistinguished from the above PTFE for molding. Such low molecular weightPTFE powders are often used to modify the properties of otherthermoplastic resins, thermosetting resins, coatings, inks, oils, etc.by blending, by making use of the excellent properties of PTFE, forexample, lubricity. In connection with the properties of the lowmolecular weight PTFE, it is known that such PTFE has flowability in amelt state, and its molded articles are brittle and do not havesufficient mechanical strength for the practical applications.

As explained above, a PTFE fine powder is mainly shaped by a pasteextrusion method in which a liquid lubricant is compounded in the PTFEfine powder and the compound is extruded with an extruder in the form ofa rod or a tube. The extruded articles can be used as such, or rolled toform a sheet, and used as a sealing material in a non-sintered state.Alternatively, they are sintered and used as molded articles such astubes, wire-coatings, etc.

Since PTFE fine powders are supplied to such special molding methods andapplications, they are required to have high productivity in the pasteextrusion molding process, and rolling and expansion processability. Inaddition, they are required to have sinterability and dimensionalstability during sintering. Sintered molded articles are required tohave good mechanical properties and transparency. It is known that allthe required properties are largely governed by the properties of thePTFE fine powders used as raw materials.

However, such required properties cover a wide variety of propertiesfrom the shaping properties of the PTFE fine powders to the propertiesof the final molded articles. In general, conventional PTFE finepowders, which have been developed, may cost some properties to improveother properties. For example, some PTFE fine powders have goodmoldability, but the molded articles made of such PTFE fine powders havelow mechanical properties. When the PTFE fine powders are evaluated fromthe viewpoint of moldability, some have good paste extrusionmoldability, but low rolling properties or expandability.

Thus, many proposals have been made to improve the properties of PTFEfine powders.

For example, a method for improving PTFE while maintaining its inherentnon-melt processability is known, which method comprises copolymerizingabout 1 wt. % or less of other fluorine-containing monomer as a modifierwith tetrafluoroethylene (TFE). The copolymer obtained by such a methodis named modified PTFE and differentiated from melt-processable TFEcopolymers (see JP-B-37-4643, JP-B-50-38159 (=U.S. Pat. No. 3,819,594)and JP-B-56-26242).

JP-B-37-4643 and JP-B-56-26242 disclose a method for improving pasteextrudability at a high reduction ratio (HRR. RR (reduction ratio)=aratio of the cross section of the cylinder of an extruder for pasteextrusion into which a paste is charged to the cross section of theoutlet of an extrusion die). However, the disclosed PTFE fine powdercannot be used in applications which require heat resistance at hightemperature, since it provides molded articles having low heatstability.

JP-B-50-38159 discloses the production of fluoroalkyl vinylether-modified PTFE fine powder having a low standard specific gravityand a low melt viscosity, and describes that such modification canimprove the mechanical properties, in particular, flexural life of thepolymer.

JP-A-64-1711 discloses fluoroalkyl vinyl ether-modified PTFE fine powderhaving a low standard specific gravity and a high melt viscosity, anddescribes that the rollability and expandability of the polymer can beimproved.

Furthermore, JP-A-7-165828 (=U.S. Pat. No. 5,641,571) discloses a PTFEmicropowder comprising TFE and about 3 wt. % or less of acopolymerizable monomer, which has a polydispersibility of 1.5 to 2.5 interms of Mw/Mn, a specific surface area (BET) of 7 to 13 m²/g and aprimary particle size of 150 to 250 nm. This micropowder has flowabilityin a melt state and is used as a modifier. Thus, it is the same type ofa material as the above-described waxes (low molecular weight PTFE).

Modified PTFE fine powders are mostly processed in the form of a wirecoating or a tube, and used in fields where particularly high quality isrequired, for example, aircraft, automobiles, chemical industries, etc.In particular, tubes are used as flexible piping in hydraulic controlsystems, fuel supply pipes, high pressure steam pipes, chemical liquidtransfer hoses, etc. In these applications, liquid leakage caused by thebreakage of tubes should be avoided from the viewpoint of safety. Thus,it is desired to improve the mechanical properties, in particular,pressure resistance of the tubes.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a modified PTFE finepowder, which has good paste exrudability and transparency, and canprovide molded articles having good mechanical properties, inparticular, excellent pressure resistance.

Another object of the present invention is to provide a method for theproduction of such a PTFE fine powder.

To achieve the above objects, the present invention provides a modifiedPTFE fine powder comprising agglomerates of colloidal particles with anaverage particle size of from 0.05 to 0.5 μm which comprise a copolymerof tetrafluoroethylene and at least one olefin monomer selected from thegroup consisting of a fluoroalkyl vinyl ether of the general formula:

X(CF₂)_(n)OCF═CF₂  (I)

wherein X is a hydrogen atom, a fluorine atom or a chlorine atom, and nis an integer of 1 to 6, and a fluoroalkyl vinyl ether of the generalformula:

C₃F₇(OCF₂CF₂CF₂)_(m)[OCF(CF₃)CF₂]_(p)OCF═CF₂  (II)

wherein m and p are independently an integer of 0 to 4, provided thatthe sum of m and p is not 0 (zero), preferably C₃F₇OCF═CF₂, the contentof the olefin monomer being from 0.02 to 0.3 wt. %, preferably from 0.03to 0.2 wt. %, wherein the copolymer has a molecular weight distributionMw/Mn of from 1.5 to 4.5, preferably from 2.0 to 4.0 and a standardspecific gravity of from 2.135 to 2.175, preferably from 2.140 to 2.160.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a tool which is used in the measurement ofthe burst pressure of a sample in Examples.

FIG. 1A is a cross section prior to the fitting of a sample to the tool,and FIG. 1B is a cross section during the measurement.

DETAILED DESCRIPTION OF THE INVENTION

The modified PTFE fine powder of the present invention may be preparedby a polymerization method in which a gaseous chain transfer agent ispresent in a polymerization system when tetrafluoroethylene iscopolymerized with at least one olefin monomer selected from the groupconsisting fluoroalkyl vinyl ethers of the above general formulas (I)and (II), in an aqueous medium containing 0.02 to 0.3 wt. % of awater-soluble fluorine-containing dispersant.

In the modified PTFE fine powder of the present invention, it isimportant to use the above fluoroalkyl vinyl ether (I) and/or (II) as amodifier. The use of other known modifiers, for example,hexafluoropropene or chlorotrifluoroethylene cannot achieve the effectsof the present invention.

The content of the fluoroalkyl vinyl ether (I) and/or (II) in thecopolymer is from 0.02 to 0.3 wt. %, preferably from 0.03 to 0.2 wt. %based on the weight of the copolymer. When the content of thefluoroalkyl vinyl ether is less than 0.02 wt. %, the copolymer has lowpaste extrudability at HRR, sinterability and transparency. When thecontent exceeds 0.3 wt. %, a reaction rate decreases, and thus theproductivity of the copolymer deteriorates.

The characteristics of the modified PTFE fine powder of the presentinvention are the narrower molecular weight distribution Mw/Mn thanthose of the conventional modified PTFE fine powders, and a smallstandard specific gravity (SSG). These characteristics contribute to thegood mechanical properties of molded articles.

In general, the molecular weight distribution of a polymer is expressedby the ratio of a weight average molecular weight Mw to a number averagemolecular weight Mn. The measurement of the molecular weightdistribution is explained by S. Wu in Polymer Engineering & Science,1988, Vol. 28, 538 and 1989, Vol. 29, 273.

A molecular weight distribution Mw/Mn is usually from 1.5 to 4.5 in thepresent invention. When the molecular weight distribution exceeds 4.5,molded articles have low pressure resistance. When the molecular weightdistribution is less than 1.5, a large amount of a chain transfer agentshould be present in a polymerization system, land thus a reaction ratedecreases and the productivity of a polymer deteriorates. A preferredmolecular weight distribution is in the range between 2.0 and 4.0, inparticular, between 2.6 and 4.0.

It is known that a standard specific gravity has a correlation with amolecular weight of PTFE, and a smaller standard specific gravity meansa larger molecular weight. Thus, a standard specific gravity is used asthe yardstick of a molecular weight.

In general, a high molecular weight is preferable for mechanicalproperties. The standard specific gravity of modified PTFE havingsufficient mechanical properties for practical applications is 2.175 orless. The amount of a polymerization initiator should be very small toproduce modified PTFE having a standard specific gravity of less than2.135 (namely a very high molecular weight). Such a very small amount ofan initiator is unpreferable from the economical point of view, since apolymerization rate greatly decreases. The standard specific gravity ofthe modified PTFE fine powder is usually from 2.135 to 2.175, preferablyfrom 2.140 to 2.160.

The modified PTFE fine powder of the present invention comprisesagglomerates of colloidal particles having a number average particlesize of from 0.05 to 0.5 μm. When a number average particle size is lessthan 0.05 μm, a pressure in the paste extrusion process largelyincreases, and thus the molding at HRR becomes difficult. When a numberaverage particle size exceeds 0.5 μm, a latex has low sedimentationstability, which is less preferable for the production of a fine powder.

The modified PTFE fine powder of the present invention may be producedby the following method.

Chain transfer agents used in the polymerization of TFE may be hydrogen;hydrocarbons such as methane, ethane, propane, butane, etc.;halohydrocarbons such as CH₃Cl, CH₂Cl₂, CH₂CF₂, etc.; and water-solubleones such as methanol, ethanol, etc. Chain transfer agents which are ina gas state under conventional polymerization conditions are preferredto control the molecular weight and molecular weight distribution ofmodified PTFE.

Examples of the gaseous chain transfer agents are hydrogen; hydrocarbonssuch as methane, ethane, propane, butane, etc.; and halohydrocarbonssuch as CH₃Cl, CH₂CF₂, etc. Among them, methane, ethane and propane arepreferable, and methane and ethane are particularly preferable. Methaneand ethane can function as chain transfer agents in a small amount, andthey do not decrease a polymerization rate greatly.

The amount of a chain transfer agent is usually from 1 to 1000 ppm,preferably from 1 to 500 ppm based on the whole TFE monomer in apolymerization system.

A polymerization initiator may be any one that is used in theconventional polymerization methods of TFE. Examples of thepolymerization initiator include persulfates such as ammoniumpersulfate, potassium persulfate, etc. and water-soluble organicperoxides such as disuccinic acid peroxide, diglutaric acid peroxide,etc. Persulfates are preferable.

The amount of a persulfate is usually from 2 to 300 ppm, preferably from2 to 200 ppm based on an aqueous medium. The amount of a polymerizationinitiator may be determined in accordance of an intended standardspecific gravity. For example, the amount of a polymerization initiatoris from 2 to 20 ppm at a polymerization temperature in the range between70 and 80° C.

A polymerization temperature may be selected from a wide range between10 and 95° C. When persulfates are used, a polymerization temperature isfrom 40 to 80° C. When a polymerization is carried out at a relativelylow temperature, reducing agents such as sulfites, acidic sulfites, etc.are used in combination with persulfates to form a redox system.

Examples of a water-soluble fluorine-containing dispersant include acompound of the general formula:

X(CF₂)_(a)COOH

wherein X is a hydrogen atom, a fluorine atom or a chlorine atom, and ais an integer of 6 to 12, a compound of the general formula:

Cl(CF₂CFCl)_(b)CF₂COOH

wherein b is an integer of 2 to 6,

a compound of the general formula:

 (CF₃)₂CF(CF₂CF₂)_(c)COOH

wherein c is an integer of 2 to 6,

a compound of the general formula:

F(CF₂)_(d)O(CFYCF₂O)_(e)CFYCOOH

wherein Y is a fluorine atom or a trifluoromethyl group, d is an integerof 1 to 5, and e is an integer of 1 to 5, and their ammonium salts oralkali metal salts (e.g. potassium salts, sodium salts).

In particular, it is preferable to use a compound of the formula:

C_(n)F_(2n+1)COOX

or

C₃F₇O[CF(CF₃)CF₂O]_(q)CF(CF₃)COOX

wherein n is an integer of 6 to 9, q is 1 or 2, and x is an ammoniumgroup or an alkali metal atom.

A water-soluble fluorine-containing dispersant may be charged into apolymerization reaction system prior to the start of a reaction all atonce, although it is possible to carry out a programmed charging asdescribed in JP-B-44-14466.

The amount of a water-soluble fluorine-containing dispersant is from0.02 to 0.3 wt. %, preferably from 0.03 to 0.2 wt. % based on the weightof an aqueous medium used in the polymerization reaction.

A fluoroalkyl vinyl ether (I) and/or (II) as a modifier according to thepresent invention can be charged in a polymerization system all at onceat the start of the reaction, or added portion by portion orcontinuously in the course of the reaction.

In some cases, a hydrocarbon having 12 or more carbon atoms, which issubstantially inactive against the reaction and in a liquid state underthe polymerization conditions, may be used as a dispersion stabilizerfor a polymerization system, in an amount of 2 to 10 wt. parts per 100wt. parts of an aqueous medium.

Furthermore, a buffer such as ammonium carbonate, ammonium phosphate,etc. may be added to adjust a pH value in the course of a reaction.

The polymerization reaction proceeds while maintaining a reactionpressure in a range between 0.6 and 3.9 MPa, preferably between 0.9 and3.0 MPa by the pressurization with TFE itself.

The polymerization reaction is terminated by stopping stirring anddischarging monomers outside the reaction system, when a polymerconcentration reaches 20 to 45 wt. %. Then, the aqueous dispersion of apolymer (which is called a polymer latex or simply a latex) is recoveredfrom a reactor, and transferred to subsequent steps, namely, acoagulation step and a drying step.

A polymer can be coagulated by diluting a polymer latex with water to apolymer concentration of 10 to 20 wt. %, optionally adjusting pH to aneutral or alkaline value, and then agitating the diluted latex in avessel equipped with an agitator more vigorously than in thepolymerization reaction. The agitation may be carried out with theaddition of coagulants such as water-soluble organic compounds (e.g.methanol, acetone, etc.), inorganic salts (e.g. potassium nitrate,ammonium carbonate, etc.), inorganic acids (e.g. hydrochloric acid,sulfuric acid, nitric acid, etc.), and the like. Alternatively, thecoagulation can continuously be carried out with an in-line mixer, andthe like.

Filler-containing PTFE fine powders, in which the filler is uniformlymixed, can be obtained by the addition of pigments for coloration orvarious fillers for the improvement of mechanical properties prior to orduring the coagulation.

Coagulated powders may be dried by any method such as the application ofvacuum, high frequency, hot air, etc. while maintaining a wet powderobtained by coagulation in a state in which the powder does notunnecessarily flow, or preferably in a static state. The friction ofparticles, in particular, at a high temperature will have undesirableinfluences on the fine powder type PTFE, because such PTFE particles areeasily fibrillated even with a small shear force and thus they lose theoriginal stable particulate structure.

The PTFE fine powder of the present invention is suitable as a moldingmaterial. Preferable examples of the application of such a PTFE finepowder include hydraulic system tubes or fuel tubes of aircraft andautomobiles, and flexible hoses for transporting chemical liquids,steam, etc. in the chemical industries.

Apart from the use as fine powders, the polymer of the present inventionmay be used in applications which utilize the latex of such a polymer.For example, nonionic surfactants are added to a latex after thereaction to stabilize the latex, and then the latex is concentrated.Optionally, organic or inorganic fillers are added to formulate acoating. The coating is applied onto the surface of a metal or a ceramicto obtain a coating surface which has non-tackiness and a low frictioncoefficient, and also good gloss, smoothness, wear resistance, weatherresistance and heat resistance. Such a coating is suitable for coatingrolls or cookware, impregnation of glass cloth, and the like.

Herein, polymer latexes and PTFE fine powders are anaylyzed and testedas follows:

1) Polymer Concentration

Ten grams of a polymer latex were sampled on a laboratory dish, anddried at 150° C. for about 3 hours to evaporate the medium to dryness.The residual solid was weighed, and then a polymer concentration iscalculated from the latex weight and the solid weight.

2) Number Average Particle Size

A calibration curve is prepared using a known sample. That is, thetransmittance of an incident light of 550 nm per a unit path length of apolymer latex, which has been diluted with water to a solid content of0.15 wt. %, is measured, and also a number-based length average particlesize is determined by measuring diameters of particles in a certaindirection with a transmission electron microscope. Then, a calibrationcurve is drawn using the light transmission and the average particlesize.

The average particle size of a certain sample is read from thecalibration curve using a transmittance which is measured with thecertain sample under the same conditions as above.

3) Content of a Modifier

As the content of a fluoroalkyl vinyl ether in a polymer, a value (wt.%) is used, which is obtained by multiply a ratio of an absorptionintensity at 995 cm⁻¹ to that at 935 cm⁻¹ in an IR absorption band, by0.14.

4) Standard Specific Gravity (SSG)

SSG is measured by a water-replacement method using a sample which isprepared according to ASTM D4895-89.

5) Molecular Weight Distribution Mw/Mn

A viscoelastic meter RDS-2 (manufactured by Rheometrix) is used as ameasuring apparatus, and a dynamic viscoelasticity is measured at 380°C. A frequency range is between 0.001 and 500 rad/sec., and the samplingfrequencies of measured values are 5 points in one figure at alogarithmically equal interval. The measured values are data-processedby the method of S. Wu (Polymer Engineering & Science, 1988, Vol. 28,538 and 1989, Vol. 29, 273) to obtain Mw, Mn and Mw/Mn. In this case, atime t is equal to 1/ω, and G(t) is equal to G′ (ω) in which ω is afrequency, G(t) is a relaxation modulus, and G′(ω) is a storage modulus.

The measurements are repeated until the average deviation of G′ (ω) ateach measuring frequency becomes less than 5% in two successivemeasurements.

6) Pressure Resistance Test (Measurement of Breakage Pressure)

A PTFE fine powder (90 g) is filled in a cylindrical mold having adiameter of 50 mm in an atmosphere which is maintained in a temperaturerage between 23 and ₂₅° C., and the upper part of the powder is leveled.Then, a pressure is gradually increased, and the powder was compressedunder 29.4 MPa for 2 minutes. Thereafter the compressed powder wasremoved from the mold to obtain a premolded article. The premoldedarticle is sintered at 370° C. for 90 minutes in an air-circulatingfurnace, and then cooled to 250° C. at a cooling rate of 60° C./hr.After maintaining the article at 250° C. for 30 minutes, it is removedfrom the furnace, and allowed to cool to room temperature.

This article is cut to form a film having a thickness of 0.50 mm, andannealed at 380° C. for 5 minutes in an air-circulating furnace. Then,the film is cooled to 250° C. at a cooling rate of 10° C./min. andmaintained at 250° C. for 5 minutes, following by allowing to cool toroom temperature.

The annealed film sample is attached to a tool shown in FIG. 1, andnitrogen gas is blown in an atmosphere of 0° C. to pressurize thesample. The pressure is quickly raised to 3.43 MPa, and maintained at3.43 MPa for 60 seconds. Thereafter, the pressure is increased by 0.098MPa, and each pressure is maintained for 60 seconds. Thus, a pressure atwhich the film is broken is recorded. The measurement is repeated threetimes, and three pressures at break are averaged and used as a breakagepressure.

7) Paste Extrusion Test (RR: 1500)

A PTFE fine powder (50 g) and a hydrocarbon oil (ISOPAR E manufacturedby Exxon Chemical Co., Ltd.) (9.2 g) as an extrusion aid are mixed in aglass bottle and aged at room temperature (25±2° C.) for one hour. Then,the mixture is charged in an extrusion die having a reduction angle of30 degrees and an orifice of 0.57 mm in inner diameter and 1.95 mm inland-length at its lower end, which die is equipped with a cylinderhaving an inner diameter of 25.4 mm. Then, a load of 5.7 MPa is appliedto a piston inserted in the cylinder, and maintained for one minute.Immediately after that, the above mixture is extruded through theorifice at room temperature at a ram speed of 20 mm/min. to obtain arod. A pressure in an interval in which the pressure is equilibrated inthe latter extrusion period is divided by the cross sectional area ofthe cylinder is used as an extrusion pressure (MPa).

EXAMPLES Example 1

In a 6 liter stainless-steel (SUS 316) autoclave equipped withstainless-steel (SUS 316) anchor type agitation blades and atemperature-regulating jacket, deionized water (2980 ml), liquidparaffin (a first class grade reagent, manufactured by KISHIDA ChemicalCo., Ltd.) (120 g) and ammonium perfluorooctanoate (3.0 g)were charged,and the internal atmosphere was replaced with nitrogen gas three timesand with TFE gas two times while maintaining the autoclave at 70° C. topurge oxygen. Then, ethane gas (1 cc) was charged, and the internalpressure was raised to 1.52 MPa with the TFE gas. The mixture wasstirred at 280 rpm, and the internal temperature was maintained at 70°C.

After that, perfluoropropyl vinyl ether (PPVE) (2.8 g) and then thesolution of ammonium persulfate (12 mg) dissolved in water (20 ml) werecharged under the pressure of TFE to raise the internal pressure of theautoclave to 1.57 MPa. Thus, the reaction acceleratively proceeded, butthe reaction temperature was maintained at 70° C., and the stirring ratewas kept at 280 rpm. TFE was continuously supplied to maintain theinternal pressure of the autoclave at 1.57±0.05 MPa.

The reaction was terminated by stopping the stirring and supply of themonomer, and immediately discharging the gas in the autoclave down to anatmospheric pressure, when 1219 g of the TFE monomer was consumed.

The total reaction time was 17.2 hours, and the average particle sizewas 0.18 μm. The polymer content of the obtained latex was 28.9 wt. %.

The obtained latex was coagulated, and the coagulated polymer waswashed. Then, the polymer powder was dried at 140° C. for 18 hours. Withthe obtained fine powder, a PPVE content in the polymer was measured,and it was 0.140 wt. %. The polymer had a SSG of 2.165, and a molecularweight distribution Mw/Mn of 2.72.

The breakage pressure of the molded film was 5.48 MPa, and the pressureresistance was good. The paste extrusion pressure at a RR of 1500 was141 MPa, and the continuous molded article was produced.

Example 2

In the same autoclave as that used in Example 1, deionized water (2980ml), liquid paraffin (a first class grade reagent, manufactured byKISHIDA Chemical Co., Ltd.) (120 g) and ammonium perfluorooctanoate (3.0g) were charged, and the internal atmosphere was replaced with nitrogengas three times and with TFE gas two times while maintaining theautoclave at 55° C. to purge oxygen. Then, ethane gas (2 cc) wascharged, and the internal pressure was raised to 1.03 MPa with the TFEgas. The mixture was stirred at 280 rpm, and the internal temperaturewas maintained at 55° C.

After that, PPVE (2.5 g) and then the solution of ammonium persulfate(21 mg) dissolved in water (20 ml) were charged under the pressure ofTFE to raise the internal pressure of the autoclave to 1.08 MPa. Thus,the reaction acceleratively proceeded, but the reaction temperature wasmaintained at 55° C., and the stirring rate was kept at 280 rpm. TFE wascontinuously supplied to maintain the internal pressure of the autoclaveat 1.08±0.05 MPa.

The reaction was terminated by stopping the stirring and supply of themonomer, and immediately discharging the gas in the autoclave down to anatmospheric pressure, when 1348 g of the TFE monomer was consumed.

The total reaction time was 27.3 hours, and the average particle sizewas 0.20 μm. The polymer content of the obtained latex was 31.0 wt. %.

The obtained latex was post-processed in the same manners as in Example1 to obtain a fine powder. A PPVE content in the polymer was 0.115 wt.%. The polymer had a SSG of 2.153, and a molecular weight distributionMw/Mn of 3.89.

The breakage pressure of the molded film was 5.80 MPa, and the pressureresistance was good. The paste extrusion pressure at a RR of 1500 was111 MPa, and the continuous molded article was produced.

Example 3

In the same autoclave as that used in Example 1, deionized water (2980ml), liquid paraffin (a first class grade reagent, manufactured byKISHIDA Chemical Co., Ltd.) (120 g) and ammonium perfluorooctanoate (3.0g) were charged, and the internal atmosphere was replaced with nitrogengas three times and with TFE gas two times while maintaining theautoclave at 50° C. to purge oxygen. Then, ethane gas (10 cc) wascharged, and the internal pressure was raised to 1.03 MPa with the TFEgas. The mixture was stirred at 280 rpm, and the internal temperaturewas maintained at 50° C.

After that, PPVE (3.0 g) and then the solution of ammonium persulfate(45 mg) dissolved in water (20 ml) were charged under the pressure ofTFE to raise the internal pressure of the autoclave to 1.08 MPa. Thus,the reaction acceleratively proceeded, but the reaction temperature wasmaintained at 50° C., and the stirring rate was kept at 280 rpm. TFE wascontinuously supplied to maintain the internal pressure of the autoclaveat 1.08±0.05 MPa.

The reaction was terminated by stopping the stirring and supply of themonomer, and immediately discharging the gas in the autoclave down to anatmospheric pressure, when 1404 g of the TFE monomer was consumed.

The total reaction time was 30.4 hours, and the average particle sizewas 0.19 μm. The polymer content of the obtained latex was 31.9 wt. %.

The obtained latex was post-processed in the same manners as in Example1 to obtain a fine powder. A PPVE content in the polymer was 0.149 wt.%. The polymer had a SSG of 2.161, and a molecular weight distributionMw/Mn of 2.10.

The breakage pressure of the molded film was 5.52 MPa, and the pressureresistance was good. The paste extrusion pressure at a RR of 1500 was125 MPa, and the continuous molded article was produced.

Example 4

In the same autoclave as that used in Example 1, deionized water (2980ml), liquid paraffin (a first class grade reagent, manufactured byKISHIDA Chemical Co., Ltd.) (120 g) and ammonium perfluorooctanoate (3.0g) were charged, and the internal atmosphere was replaced with nitrogengas three times and with TFE gas two times while maintaining theautoclave at 70° C. to purge oxygen. Then, ethane gas (2 cc) wascharged, and the internal pressure was raised to 2.69 MPa with the TFEgas. The mixture was stirred at 280 rpm, and the internal temperaturewas maintained at 70° C.

After that, PPVE (5.0 g) and then the solution of ammonium persulfate(30 mg) dissolved in water (20 ml) were charged under the pressure ofTFE to raise the internal pressure of the autoclave to 2.74 MPa. Thus,the reaction acceleratively proceeded, but the reaction temperature wasmaintained at 70° C., and the stirring rate was kept at 280 rpm. TFE wascontinuously supplied to maintain the internal pressure of the autoclaveat 2.74±0.05 MPa.

The reaction was terminated by stopping the stirring and supply of themonomer, and immediately discharging the gas in the autoclave down to anatmospheric pressure, when 1425 g of the TFE monomer was consumed.

The total reaction tide was 2.2 hours, and the average particle size was0.18 μm. The polymer content of the obtained latex was 32.2 wt. %.

The obtained latex was post-processed in the same manners as in Example1 to obtain a fine powder. A PPVE content in the polymer was 0.132 wt.%. The polymer had a SSG of 2.152, and a molecular weight distributionMw/Mn of 3.04.

The breakage pressure of the molded film was 5.38 MPa, and the pressureresistance was good. The paste extrusion pressure at a RR of 1500 was154 MPa, and the continuous molded article was produced.

Comparative Example 1

The following experiment was carried out according to the methoddescribed in JP-B-50-38159.

In the same autoclave as that used in Example 1, deionized water (2980ml), liquid paraffin (a first class grade reagent, manufactured byKISHIDA Chemical Co., Ltd.) (120 g) and ammonium perfluorooctanoate (3.0g) were charged, and the internal atmosphere was replaced with nitrogengas three times and with TFE gas two times while maintaining theautoclave at 70° C. to purge oxygen. Then, the internal pressure wasraised to 2.69 MPa with the TFE gas. The mixture was stirred at 280 rpm,and the internal temperature was maintained at 70° C.

After that, PPVE (5.0 g), and then the solution of ammonium persulfate(30 mg) dissolved in water (20 ml) were charged under the pressure ofTFE to raise the internal pressure of the autoclave to 2.74 MPa. Thereaction temperature was maintained at 70° C., and the stirring rate waskept at 280 rpm. TFE was continuously supplied to maintain the internalpressure of the autoclave at 2.74±0.05 MPa.

The reaction was terminated by stopping the stirring and supply of themonomer, and immediately discharging the gas in the autoclave down to anatmospheric pressure, when 1292 g of the TFE monomer was consumed.

The total reaction time was 1.9 hours, and the average particle size was0.18 μm. The polymer content of the obtained latex was 30.1 wt. %.

The obtained latex was post-processed in the same manners as in Example1 to obtain a fine powder. A PPVE content in the polymer was 0.127 wt.%. The polymer had a SSG of 2.149, and a large molecular weightdistribution Mw/Mn of 5.00. Thus, the pressure resistance was low.

Comparative Example 2

The following experiment was carried out according to the methoddescribed in JP-A-64-1711.

In the same autoclave as that used in Example 1, deionized water (2960ml), liquid paraffin (a first class grade reagent, manufactured byKISHIDA Chemical Co., Ltd.) (120 g) and ammonium perfluorooctanoate (3.0g) were charged, and the internal atmosphere was replaced with nitrogengas three times and with TFE gas two times while maintaining theautoclave at 70° C. to purge oxygen. Then, the internal pressure wasraised to 2.69 MPa with the TFE gas. The mixture was stirred at 280 rpm,and the internal temperature was maintained at 70° C.

After that, PPVE (5.0 g) and then the solution of ammonium persulfate(51 mg) dissolved in water (20 ml) were charged under the pressure ofTFE to raise the internal pressure of the autoclave to 2.74 MPa. Thereaction temperature was maintained at 70° C., and the stirring rate waskept at 280 rpm. TFE was continuously supplied to maintain the internalpressure of the autoclave at 2.74±0.05 MPa.

When the amount of TFE consumed in the reaction reached 640 g, thesolution of hydroquinone (25.6 mg) dissolved in water (20 ml) wascharged under the pressure of TFE. The reaction was terminated bystopping the stirring and supply of the monomer, and immediatelydischarging the gas in the autoclave down to an atmospheric pressure,when 1280 g of the TFE monomer was consumed.

The total reaction time was 4.1 hours, and the average particle size was0.19 μm. The polymer content of the obtained latex was 29.9 wt. %.

The obtained latex was post-processed in the same manners as in Example1 to obtain a fine powder. A PPVE content in the polymer was 0.118 wt.%. The polymer had a SSG of 2.155, and a large molecular weightdistribution Mw/Mn of 6.80. Thus, the pressure resistance was low.

TABLE 1 Example No. C. Example No. (unit) 1 2 3 4 1 2 Reaction 1.57 1.081.08 2.74 2.74 2.74 pressure (MPa) Reaction temp. 70 55 50 70 70 70 (°C.) Charged 1 2 10 1 0 0 amount of ethane (cc) Charged 2.8 2.5 3.0 5.05.0 5.0 amount of PPVE (g) Charged 12 21 45 30 30 51 amount of APS (mg)Reaction time 17.2 27.3 30.4 2.2 1.9 4.1 (hrs) Polymer 28.9 31.0 31.932.2 30.1 29.9 concentration (wt. %) Number 0.18 0.20 0.19 0.18 0.180.19 average particle size (μm) PPVE content 0.140 0.115 0.149 0.1320.127 0.118 (wt. %) SSG 2.165 2.153 2.161 2.152 2.149 2.155 Mw/Mn 2.723.89 2.10 3.04 5.00 6.80 Breakage 5.48 5.80 5.52 5.38 4.99 4.60 pressure(MPa) Extrusion 141 111 125 154 148 136 pressure (MPa)

What is claimed is:
 1. A modified polytetrafluoroethylene fine powdercomprising agglomerates of colloidal particles with an average particlesize of from 0.05 to 0.5 μm which comprise a copolymer oftetrafluoroethylene and at least one olefin monomer selected from thegroup consisting of a fluoroalkyl vinyl ether of the general formula:X(CF₂)_(n)OCF═CF₂ wherein X is a hydrogen atom, a fluorine atom or achlorine atom, and n is an integer of 1 to 6, and a fluoroalkyl vinylether of the general formula:C₃F₇(OCF₂CF₂CF₂)_(m)[OCF(CF₃)CF₂]_(p)OCF═CF₂ wherein m and p areindependently an integer of 0 to 4, provided that the sum of m and p isnot 0 (zero), a content of said olefin monomer being from 0.02 to 0.3wt. %, wherein said copolymer has a molecular weight distribution Mw/Mnof from 1.5 to 4.5 and a standard specific gravity of from 2.135 to2.175.
 2. A modified polytetrafluoroethylene fine powder according toclaim 1, wherein said molecular weight distribution Mw/Mn is from 2.0 to4.0.
 3. A modified polytetrafluoroethylene fine powder according toclaim 1, wherein said standard specific gravity is from 2.140 to 2.160.4. A modified polytetrafluoroethylene fine powder according to claim 1,wherein said fluoroalkyl vinyl ether is a compound of the generalformula: X(CF₂)_(n)OCF═CF₂ wherein X is a hydrogen atom, a fluorine atomor a chlorine atom, and n is an integer of 1 to
 6. 5. A method forpreparing a modified polytetrafluoroethylene fine powder as claimed inclaim 1 comprising copolymerizing tetrafluoroethylene and at least oneolefin monomer selected from the group consisting of a fluoroalkyl vinylether of the general formula: X(CF₂)_(n)OCF═CF₂ wherein X is a hydrogenatom, a fluorine atom or a chlorine atom, and n is an integer of 1 to 6,and a fluoroalkyl vinyl ether of the general formula:C₃F₇(OCF₂CF₂CF₂)_(m)[OCF(CF₃)CF₂]_(p)OCF═CF₂ wherein m and p areindependently an integer of 0 to 4, provided that the sum of m and p isnot 0 (zero), in an aqueous medium containing 0.02 to 0.3 wt. % of awater-soluble fluorine-containing dispersant, wherein a gaseous chaintransfer agent is present in the polymerization system.
 6. A methodaccording to claim 5, wherein said chain transfer agent is hydrogen,methane, ethane or propane.
 7. A method according to claim 5, wherein apolymerization initiator is ammonium persulfate or an alkali metalpersulfate, and a reaction temperature is from 40 to 80° C.